1. Field of the Invention
The present invention relates to a method to form uniform or heterogeneous, straight or curved and size-controllable nanostructures using nanochannel templates. More specifically, this method may be used to form devices and systems utilizing nanowires.
2. Discussion of the Related Art
One-dimensional metallic and semiconducting nanowires have attracted much attention recently owing to their fundamental and technological importance. Much of this attention is focused on Si nanowires (SiNW) since, at this scale, Si can become a direct-band-gap material. In the article, by B. Marsen, et al., “Fullerene-Structured Nanowires of Silicon,” Phys. Rev., V. 60, No. 16 (1999), it was suggested that silicon wires less than 100 nm in diameter may be used for developing one-dimensional quantum-wire high-speed field effect transistors and light-emitting devices with extremely low power consumption.
In a Vapor-Liquid-Solid (VLS) reaction, metal droplets (or slugs), for example, Au in the case of Si, are generally used as the mediating solvent. More specifically, gold is used in the case of silicon because Au and Si form molten eutectic alloy droplets at relatively low temperatures. In the general VLS case, a liquid alloy acts as a preferred sink or catalyst for arriving vapor, plasma, or gas precursor atoms, radicals or molecules present in a controlled ambient. This liquid sink then excretes the solid nanowire.
In recent years, many efforts have been made to synthesize nanowires by employing different methods. Most of these methods are based on a vapor-liquid-solid (VLS) growth mechanism, which was first introduced in article by R. S. Wagner, et al., Vapor-Liquid-Solid Mechanism of Single Crystal Growth, Appl. Phys. Lett. 4 (1964). In the case of silicon nanowires, the arriving Si precursor causes the Au eutectic liquid to become supersaturated with Si, and the Si precursor precipitates out at a solid-liquid interface. The Si in the vapor phase continues to diffuse into the liquid alloy droplets while precipitating and bonding to the solid Si at the liquid-solid interface. By a continuation of this process, the standard SiNW VLS approach results in an alloy droplet, which becomes displaced from the substrate and ‘rides’ atop the growing whisker.
It remains a challenge to grow composition-controllable, orientation-controllable and size-controllable nanowires. It is also difficult to assemble the wires into rational device geometries and enable current flow between nanoscale and microscale structures.
One related art solution is the growth of nanowires in disposable templates. These techniques are being explored by a number of university research groups. One of these groups is Professor Joan Redwing's research group at Pennsylvania State University. The work of that group is specifically focused on silicon nanowires, and their uses in commercially available alumina templates. More specifically, the group has researched nanowires that are used after being freed by template dissolution and the group has demonstrated the fabrication of SiNWs using a combination of this disposable-template directed synthesis and VLS growth in an article by, Lew, K. K.; Redwing, J. M., J. Cryst. Growth 14, 254 (2003). The use of the alumina membranes as the templates (e.g., dissolved/etched away templates) may provide control over nanowire diameter and length while also enabling the production of single crystal material, however, there are many disadvantages to current approaches. For example,                1. Due to the use of pre-formed commercial templates, the diameter of nanowires produced with this method is relatively large (usually 200 nm) and precludes the opportunity for nanoscale phenomena such as quantum confinement;        2. When commercially available anodic alumina membranes are employed, the pore (template channel) diameters in these materials are not precisely controlled;        3. The templates used are an array of pores (channels) in an Al2O3 membrane. These channels are not very straight or uniform;        4. To initiate SiNW growth, for example, in these channels, electrodeposition is used to place a gold slug into the center (mid-length) of the pores. To achieve this, Ag is first electrodeposited, then the Au is deposited and finally the Ag is etched away. All of these steps use electroplating and make the process complicated;        5. The alumina template is disposed of (etched away) in this approach and cannot be integrated with device assembly. After completing nanowire formation, the template must be etched away to free and then use the nanowires. It is very difficult to separate, move, electrically contact, if needed, and assemble these freed nanowires; and        6. The process is not compatible with on-chip fabrication and assembly, and requires post synthesis assembly of nanostructures to form more complicated devices.        